Optical imaging systems, and more specifically objective optical systems, provide information about a scene being viewed by collecting light (radiation) from an object space and delivering it to image space where a real image is formed and sensed by a detector. While many objective optical systems operate over a select range of wavelengths (“waveband”), imaging over multiple wavebands can provide additional information about the object space. The advent of high-performance electronic image sensors capable of sensing multiple wavebands of light enables such multi-waveband objective optical systems. However, this places additional demands on the objective optical system because it must be well-corrected over the multiple wavebands.
The design of refractive objective optical systems having multi-waveband imaging capability is inhibited by the limited availability of refractive materials able to sufficiently transmit light and enable aberration correction, particularly chromatic aberrations. Other difficulties include managing system thermal sensitivity, which causes image defocus and loss of aberration correction at elevated and depressed temperatures.
Reflective objective optical systems have the advantage that mirrors do not depend on the transmission of light, have no chromatic aberrations and tend to be less thermally sensitive. One type of reflective objective optical system employing three mirrors is referred to as a “three mirror anastigmat” or “TMA”. The mirrors of a TMA are configured so that the light traverses the system in four directions, thereby providing an unobscured view of the imaged scene, i.e., there is no vignetting.
TMAs having rotationally symmetrical mirrors that are decentered and tilted with respect to one another in one plane have been designed. However, conventional TMAs have limited compactness, relatively slow f/number, and marginal aberration correction. In addition, to meet other design requirements, such as having no signature augmentation and/or low distortion, the aperture needs to be significantly relaxed to achieve high performance. Furthermore, when a TMA is required to operate at shorter wavelengths (e.g., the visible waveband), the aberration correction becomes more difficult to attain in a compact design. Additional mirrors can be employed, but at the expense of greater complexity and perhaps greater size and weight.